Malaria is vector-borne disease infecting 200-300 million people per year worldwide. It is caused by infection of red blood cells by any one of four unicellular parasites from the genus Plasmodium. When caused by Plasmodium falciparum, malaria is life threatening, resulting in 1-2 million deaths, primarily in young children. Current treatments suffer from increasing resistance among parasites and efforts to develop a vaccine are severely limited by the Plasmodium life cycle; therefore, new treatments are needed. The recently sequenced genome of the most lethal malarial parasite, Plasmodium falciparum, revealed two adenylyl cyclase genes (PfACs) related to the bicarbonate responsive soluble adenylyl cyclases (sAC) in mammals and cyanobacteria. Their sequences predict the PfACs to be bicarbonate responsive, and we hypothesized that at least one is responsible for Plasmodium's absolute dependence on high levels of carbon dioxide/bicarbonate for viability. Therefore, the PfACs may represent new targets for an antimalarial drug. We treated P. falciparum with a small molecule (KH7) we developed which inhibits all sAC-like cyclases tested thus far, including sAC-like cyclases found in mammals, unicellular eukaryotes and bacteria. Parasites cultured in red blood cells were killed by KH7 in a dose dependent manner within a single cell cycle. A structurally related compound (KH7.15) which is inert towards sAC-like cyclases had no effect on parasite growth. In this grant application, we propose to clone and characterized the two PfAC cyclases and confirm whether they are the target of KH7 lethality. If the PfACs are validated as targets for new antimalarial drugs, we propose to use purified Plasmodium cyclases to screen a chemical library to identify compounds selective for Plasmodium cyclases relative to human sAC.